Field of the Invention
This disclosure relates generally to a housing for solid, fluidal substance for removing an undesired respiratory gas component of a respiratory gas flow and an arrangement for ventilating lungs of a subject.
Description of the Prior Art
Anesthesia machines are optimized for delivering anesthesia to a patient using volatile anesthetic agent liquids. In such systems, the anesthetic agent is vaporized and mixed into the breathing gas stream in a controlled manner to provide a gas mixture for anesthetizing the patient for a surgical operation. The most common volatile anesthetic agents are halogenated hydrocarbon chains, such as halothane, enflurane, isoflurane, sevoflurane and desflurane. Additionally, nitrous oxide (N2O) can be counted in this group of volatile anesthetic agents, although the high vapor pressure of nitrous oxide causes nitrous oxide to vaporize spontaneously in the high pressure gas cylinder, where it can be directly mixed with oxygen. The anesthetizing potency of nitrous oxide alone is seldom enough to anesthetize a patient and therefore another volatile agent is used to support that.
Since the volatile anesthetic agents are expensive and are effective greenhouse gases that are harmful to the atmospheric ozone layer, anesthesia machines have been developed to minimize the consumption of the gases. To keep a patient anesthetized, a defined brain partial pressure for the anesthetic agent is required. This partial pressure is maintained by keeping the anesthetic agent partial pressure in the lungs adequate. During a steady state, the lung and body partial pressures are equal, and no net exchange of the anesthetic agent occurs between the blood and the lungs. However, to provide oxygen and eliminate carbon dioxide, continuous lung ventilation is required.
Anesthesia machines are designed to provide oxygen to the patient and eliminate carbon dioxide (CO2), while preserving the anesthetic gases. To meet these goals a re-breathing circuit is used, in which a patient's exhaled gas is reintroduced for inhalation. Before re-inhalation, carbon dioxide must be removed from the gas, which is done with a carbon dioxide absorber. Before inhalation, the gas is supplied with additional oxygen and anesthetic agents based upon the patient demand. In this arrangement, the additional gas flow added to the re-breathing circuit can be less than 0.5 L/min although the patient ventilation may be 5-10 L/min. Such ventilation of the lung is carried out using a ventilator pushing inhalation gas with overpressure to the patient's lungs and then allowing that to flow out passively from the pressurized lungs to the breathing circuit.
Ventilation carries the breathing circuit oxygen to lungs for uptake to be burned in body metabolism. The outcome is CO2 that diffuses to lungs and is carried out with exhalation gas. Before re-inhalation the gas is guided through an absorber for CO2 removal. Effective CO2 removal requires close contact with the breathing gas and the removing substance. To provide large contact area, the removing substance is therefore a surface of a porous structure of granules that fill a cartridge. The form of this granular structure is guided by flow resistance, the limitation of which is one of the key design goals of the breathing circuit. In an absorber optimized for minimal resistance, the gas flow path through the stacked granules is short and the flow distributes to a wide area. In such structure the gas flows slowly because the large surface area provides time for reaction between the gas and absorbent granules.
However, such optimal wide and short cartridge design involves a problem. Because the removing material is in granules, the granules may move in relation to each other. Packaging of the granules into a cartridge occurs in a factory, and thereafter the cartridge is transported to a customer site. The granules experience shaking during transportation which compresses the granules, increases the granule packaging grade and reduces the volume of the granule bed in the cartridge. Therefore the cartridge may have some empty space on its top when used. Because of the empty space, the gas flows through the absorber vertically, since when flowing horizontally the gas, which favors the route of the least resistance, would flow through the empty space without communication with the absorbent and thus allows the CO2 to flow through the absorber.
When the gas flows vertically, the horizontal empty space is not harmful since the horizontal empty space does not disturb the internal flow resistance distribution within the cartridge. However, if the top surface of the granules is slanted as shown in FIG. 1, flow density at the low granule level, where the empty space 1 exists, increases over the areas of high level of granules 2. This is known as a flow channeling. A typical cartridge 3 comprises a gas input 4, a gas output 5 and mesh plates 6 and 7 for preventing granules therebetween to escape from the cartridge. A demand of CO2 removal is proportional to the flow rate and the absorbent is consumed faster at the volumes of high gas flow. The absorbent volume in these areas is also smaller. Each of these factors causes fast absorbent wear-out at these high flow volumes. When all absorbent has been consumed, the CO2 penetrates through the cartridge using the flow path where the capacity has been reduced, which increases the inspired patient gas CO2 concentration. This signals the wear-out of the whole cartridge, even though unused material may still exist at the reduced flow volumes. Therefore, slanting reduces usable cartridge CO2 removal capacity. The surface level of the granules may become slanted if inclined during unpacking and installing a cartridge having empty space caused during transportation.